adaptiveOnsets(): N[] array has no purpose

[dpwe]

On lines like https://github.com/spotify/echoprint-codegen/blob/master/src/Fingerprint.cxx#L128
I see logic to set N[j] to the threshold at the most recent crossing of the envelope, but I don't see anywhere where N[j] is read back or used. I conclude it's vestigial and should be removed.

[dpwe]

(in fact, if the compiler is any good, it probably has no impact on the object code anyway).